An electromagnetic fuel injector comprises a main body having a central cylindrical cavity which acts as a duct for the fuel and ends in a valve adapted to regulate the flow of fuel and provided with a moving shutter controlled by an electromagnetic actuator. The main body is made from ferromagnetic material and houses a coil of the electromagnetic actuator. A fixed armature and a moving armature of the ferromagnetic actuator are disposed in the central cavity and are made from ferromagnetic material. In operation, the fixed armature is adapted magnetically to attract the moving armature against the action of a spring in order to cause a displacement of the shutter which is mechanically rigid with this moving armature. It will be appreciated that, because a force of magnetic attraction is generated between the fixed armature and the moving armature, it is necessary for the fixed armature and the moving armature to be traversed by the magnetic flux generated by the coil.
In order to try to reduce the magnetic flux dispersed, i.e. to try to reduce the magnetic flux generated by the coil which does not impinge on the fixed armature or the moving armature, at least one insert of non-ferromagnetic material (metal or plastic) is provided in the main body and is adapted to create a barrier to the passage of the magnetic flux so as to force this magnetic flux to pass through the fixed armature and the moving armature. However, the production of the insert from non-ferromagnetic material requires special processing which substantially increases the cost of the injector; moreover, at the junctions between the insert of non-ferromagnetic material and the main body there may be leakages of fuel.
As an alternative to the above-described use of an insert of non-ferromagnetic material, it is possible appropriately to shape the main body in order to create air gap zones adapted to perform the same function of creating a barrier to the passage of the magnetic flux in order to force this magnetic flux to pass through the fixed armature and the moving armature. However, the production of these air gap zones in the main body is laborious and complex.
US2002130206 discloses a fuel injector including a tubular casing having an axial fuel passage; disposed within the fuel passage are a valve seat element, a core cylinder, and a valve element axially moveably disposed therebetween and opposed to the core cylinder with an axial air gap. An electromagnetic actuator cooperates with the casing, the valve element and the core cylinder to form a magnetic field forcing the valve element to the open position against a spring between the valve element and the core cylinder upon being energized. The casing includes a reluctance portion producing an increased magnetic reluctance and allowing the magnetic field to extend to the valve element and the core cylinder through the air gap; the reluctance portion has a reduced radial thickness and an axial length extending over the air gap.
JP2002206468 discloses an injection port, which is opened and closed by a valve element, and an armature connected to the rear end of a movable body, to which the valve element is fixed; a fixed core is arranged inside the magnetic pipe, at a position opposite to a rear end surface of the armature. The valve element is energized in the closing direction by a coil spring, and a solenoid coil is arranged outside the magnetic pipe; the magnetic pipe as a whole is formed of a magnetic material.
WO9419599 discloses a fuel injector having combination valve-armature fabricated by laser welding relatively more magnetically permeable armature element to relatively less magnetically permeable valve element. Valve element contains sealing ring and landing ring, the latter being circumferentially discontinuous because of fuel passage holes through valve element, the former being non-symmetrical so that magnetic opening force causes valve-armature to open by tilting about consistent circumferential location on valve element.